|
(11) | EP 1 475 826 A3 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||
| (54) | Nitride semiconductor wafer and method of processing nitride semiconductor wafer |
| (57) Nitride semiconductor wafers which are produced by epitaxially grown nitride films
on a foreign undersubstrate in vapor phase have strong inner stress due to misfit
between the nitride and the undersubstrate material. A GaN wafer which has made by
piling GaN films upon a GaAs undersubstrate in vapor phase and eliminating the GaAs
undersubstrate bends upward due to the inner stress owing to the misfit of lattice
constants between GaN and GaAs. Ordinary one-surface polishing having the steps of
gluing a wafer with a surface on a flat disc, bringing another surface in contact
with a lower turntable, pressing the disc, rotating the disc, revolving the turntable
and whetting the lower surface, cannot remedy the inherent distortion. The Distortion
worsens morphology of epitaxial wafers, lowers yield of via-mask exposure and invites
cracks on surfaces. Nitride crystals are rigid but fragile. Chemical/mechanical polishing
has been requested in vain. Current GaN wafers have roughened bottom surfaces, which
induce contamination of particles and fluctuation of thickness. Circular nitride wafers having a diameter larger than 45mm are made and polished. Gross-polishing polishes the nitride wafers in a pressureless state with pressure less than 60g/cm2 by lifting up the upper turntable for remedying distortion. Distortion height H at a center is reduced to H≤12µm. Minute-polishing is a newly-contrived CMP which polishes the nitride wafers with a liquid including potassium hydroxide, potassium peroxodisulfate and powder, irradiates the potassium peroxodisulfate with ultraviolet rays. The CMP-polished top surface has roughness RMS of 0.1nm≤RMS≤5nm or more favorably 0.1nm≤RMS≤0.5nm. The CMP-polished bottom surface has roughness RMS of 0.1nm≤RMS≤5000nm or more favorably 0.1nm≤RMS≤2nm. TTV is less than 10µm. |